A conventional means for improving the properties such as mechanical strength, dimensional stability, and compression creep resistance of polymers has been to combine a filler with polymer. However, the uniformity with which filler distributed in the polymer is not completely specified.
Recently, methods have been developed to improve the mechanical strength, heat deformation temperature and dimensional stability of polymer by direct melt blending of nano particles such as inorganic nano particles into the polymer. However, when inorganic particles are melt-mixed with polymer, the mutual cohesive force of the particles is found to increase as particle diameter is decreased and the inorganic particles tend to aggregate, that is, the inorganic particles cluster together, especially at when the particles are at the nano level in size, i.e. about 1 to 1000 nm in diameter. Therefore, even when the nano particles are directly melt-mixed with the polymer, it is extremely difficult to disperse the particles at the nano level in the polymer (Powder Body Engineering Handbook, 2nd Edition, p. 291-294, 1983, as reported in the Proceedings of the 47th Meeting of the Japan Society of Materials Science, Kyoto, Oct. 29-30, 2003, pp. 150-151).
One approach to overcoming the problems of the above described direct melt-mixing method, is a solution-mixing method wherein a colloidal solution of stably dispersed inorganic particles is mixed with a functionalized polymer dissolved in a liquid. For example, U.S. Pat. No. RE37,022 proposes a composition (coating agent) wherein perfluoropolymer is dissolved in an organosol wherein inorganic particles with an average particle diameter of 1000 nm or less, treated with fluorine-containing surfactant, are dispersed in a fluorinated solvent having no hydrogen (that is, the solvent has no hydrogen atoms bonded to the multivalent atoms of the solvent molecule) or a solvent made by mixing said solvent with a fluorinated solvent that contains hydrogen. The use of functionalized fluoropolymer and fluorinated solvents make this an expensive and inconvenient approach, suitable only for specialized applications.
U.S. Pat. No. 6,350,806 is directed to water-based paint, which is cured at up to 300° C., made of aqueous fluoropolymer dispersion which is added to aqueous-emulsified acrylate and methacrylate monomer, which are then polymerized. To the resulting polymer dispersion is mixed dry colloidal silica that is been treated with organoalkoxysilane. In the absence of the organoalkoxysilane, the silica is not stably mixed and the paint lacks storage stability. The distribution of the treated silica particles in the dried paint coating is not disclosed. Being a paint, the composition is not suitable for compression molded, extrusion molded, or injection molded articles. In view of the substantial acrylic content, 30 parts acrylate to 100 parts fluoropolymer, the resulting polymer composition lacks the thermal properties and oxidative resistance characteristic of fluoropolymers.
Another approach to the above described direct melt-mixing method is reported in Colloid and Surfaces, vol. 63, p. 103-111, 1992 wherein it is disclosed that aggregate is created from the solution made from mixing heterogeneous particles, wherein a colloidal silica to which potassium chloride is added so that the pH value is 5.6, is mixed with polystyrene emulsion in which the polystyrene is a copolymer that includes comonomers that provide acid and base functionality whereby the polymer is amphoteric. The silica and polystyrene particles have opposite electrical charge and thus form an unstable mixture, wherein slight mixing causes the particles to form heterocoagulates. This reference requires that the ratio of the diameter of the silica primary particles to that of the polymer primary particles be 3 or more to obtain the proper aggregate composed of a relatively large silica core and small amphoteric-modified polystyrene particles clustered around the core. These aggregates are disclosed to be useful as functional particles in industrial fields.
U.S. Patent Application Publication No. 2005/0123739 discloses dispersing dry mesoporous hydrophobic-modified fused silica into polytetrafluoroethylene dispersion, which is then coagulated, and the liquid drained, and the coagulate dried at 130° C., followed by calendering into sheet form, and sintering to improve electrical properties as printed circuit substrates.
Japanese published examined application No. Hei 7-64936 proposes a method for obtaining a powder with an average particle diameter of 3 mm wherein a suspension of silicon carbide particles with an average particle diameter of 4000 nm that has been surface-treated with an aminosilane group surfactant, is added to a fluoropolymer dispersion. Then nitric acid is added to the mixture to break the emulsion and after that, trichlorotrifluoroethane is added to the mixture to coagulate and granulate the particles thereby obtaining an powder with an average particle diameter of 3 mm.
None of the above-mentioned teachings solve the problem of providing a molded article of filled polymer where the filler is nano-sized and is uniformly dispersed as such in the polymer.